Abstract T cell exhaustion is a state of T cell function often observed under conditions of chronic exposure to antigen, for example in some chronic viral infections and in solid tumors. However, the signals that drive and maintain exhaustion in T cells are still not well understood. Numerous surface protein markers of exhausted T cells have now been identified, including the transmembrane proteins PD-1 and Tim-3, among other markers. Antibodies that block the inhibitory effects of PD-1 are now licensed by the FDA for the treatment of melanoma and some lung cancers, with many more clinical trials still underway for other indications. Development of Tim-3 as a therapeutic target is progressing, but lags behind other targets, including PD-1, among others. Despite the extensive amount of correlative evidence linking Tim-3 expression to dysfunction of T cell activation during exhaustion, little is known about the mechanisms by which Tim-3 contributes to the development and/or maintenance of exhaustion. This has been challenging, in part due to the fact that multiple promiscuous ligands to Tim-3 have been identified. In addition, putative ?blocking? antibodies to Tim-3 have not been thoroughly characterized for their effects on interactions of Tim-3 with its various ligands. Finally, we have provided evidence in multiple cell types (including T cells) that Tim-3 may actually enhance early signaling through antigen receptors, raising the question of whether some Tim-3 antibodies may actually function by actively crosslinking of Tim-3. There is therefore a need for new reagents and model systems that will allow for the more direct definition of Tim-3 function in primary T cells. Unlike PD-1 or other negative regulators of T cell activity, Tim-3 contains no motifs for recruitment of inhibitory phosphatases. Rather, our recent work suggests that Tim-3 expression actually increases signaling through pathways normally associated with positive outcomes, i.e. efficient TCR/CD3-mediated T cell activation, at least under acute conditions. Data from other groups indicate that T cell exhaustion results from chronic antigenic stimulation that extends the effector phase of T cell activation, at the expense of T cell memory. We hypothesize that signaling through the cytoplasmic tail of Tim-3 contributes to T cell exhaustion and/or the rescue of T cells in a population of exhausted T cells associated with chronic T cell activation. This hypothesis will be tested with two Specific Aims. In Aim 1, and based on our previous structure/function studies, we will generate novel mouse models for inducible knockout of Tim-3 or truncation of its cytoplasmic tail. In Aim 2, we will first determine the effects of inducible Tim-3 KO or truncation, specifically in T cells, on acute T cell responses to Listeria and LCMV; in addition, we will determine the effects of inducible, T cell-specific, KO or truncation of Tim-3 on chronic infection with LCMV-Clone 13.